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United States Patent |
5,641,996
|
Omoya
,   et al.
|
June 24, 1997
|
Semiconductor unit package, semiconductor unit packaging method, and
encapsulant for use in semiconductor unit packaging
Abstract
An improved semiconductor unit package is disclosed. This package is
implemented by a semiconductor device having an electrode pad, a substrate
having a terminal electrode, a bump electrode formed on the electrode pad,
a conductive adhesion layer with flexibility, and an encapsulating layer
formed by curing a composition the viscosity and thixotropy index of which
are below 100 Pa.multidot.s and below 1.1, respectively. Such a
composition essentially consists of (A) a resin binder that contains, for
example, a polyepoxide, an acid anhydride, and a rheology modifier and (B)
a filler. The rheology modifier is one capable of impeding interaction
between a free acid contained in the acid anhydride and a polar group at
the surface of the filler. An encapsulant with improved flowability is
used, so that the encapsulant readily flows and spreads to fill a gap
between the semiconductor device and the substrate with no air bubbles.
This achieves semiconductor unit packages with high reliability and
productivity.
Inventors:
|
Omoya; Kazunori (Osaka, JP);
Oobayashi; Takashi (Osaka, JP);
Sakurai; Wataru (Osaka, JP);
Harada; Mitsuru (Osaka, JP);
Bessho; Yoshihiro (Osaka, JP)
|
Assignee:
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Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
Appl. No.:
|
593675 |
Filed:
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January 29, 1996 |
Foreign Application Priority Data
| Jan 30, 1995[JP] | 7-012779 |
| Jun 12, 1995[JP] | 7-144373 |
Current U.S. Class: |
257/787; 257/780; 257/783; 257/E21.503; 257/E21.514; 257/E23.021; 257/E23.119 |
Intern'l Class: |
H01L 023/28 |
Field of Search: |
257/737,787,779,780,781,783,786,795,789
437/211,212,219
|
References Cited
U.S. Patent Documents
4604644 | Aug., 1986 | Beckham et al. | 437/211.
|
4999699 | Mar., 1991 | Christie et al.
| |
5121190 | Jun., 1992 | Hsiao et al.
| |
5128746 | Jul., 1992 | Pennisi et al. | 257/783.
|
5136365 | Aug., 1992 | Pennisi et al. | 257/783.
|
5194930 | Mar., 1993 | Papathomas et al. | 257/773.
|
5292688 | Mar., 1994 | Hsiao et al. | 437/211.
|
5371404 | Dec., 1994 | Juskey et al. | 257/795.
|
5436503 | Jul., 1995 | Kunitomo et al. | 257/737.
|
5442240 | Aug., 1995 | Mukerji | 257/783.
|
5450283 | Sep., 1995 | Lin et al. | 257/787.
|
5550408 | Aug., 1996 | Kunitomo et al. | 257/783.
|
Other References
Patent Abstracts of Japan, vol. 018, No. 590 (E-1628) (Nov. 10, 1994) and
JP-A-6-224259 (Aug. 12, 1994).
Patent Abstracts of Japan, vo. 016, No. 397 (E-1252) (Aug. 24, 1992) and
JP-A-4-130633 (May 1, 1992).
Database WPI, Sec. Ch. Week 8850, Derwent Publ. Ltd., London, GB and
JP-A-63-268724 (Nov. 7, 1988).
Database WPI, Sec. Ch., Week 8910, Derwent Publ. Ltd., London, GB and
JP-A-1-029416 (Jan. 31, 1989).
Database WPI, Sec. Ch., Week 9504, Derwent Publ. Ltd., London, GB and
JP-A-6-313027 (Nov. 8, 1994).
Database WPI, Sec. Ch., Week 9444, Derwent Publ. Ltd., London, GB and
JP-A-6-279654 (Oct. 4, 1994).
|
Primary Examiner: Saadat; Mahshid D.
Assistant Examiner: Clark; S. V.
Attorney, Agent or Firm: McDermott, Will & Emery
Claims
The invention claimed is:
1. A semiconductor unit package which comprises:
(a) a semiconductor device having an electrode pad;
(b) a substrate having a terminal electrode;
(c) a bump electrode formed on said electrode pad of said semiconductor
device;
(d) a conductive adhesive layer which is formed of a conductive adhesive
with flexibility and which establishes an electrical connection between
said bump electrode and said terminal electrode; and
(e) an encapsulating layer which is formed by curing a composition having a
viscosity of below 100 Pa.multidot.s and a thixotropy index of below 1.1
and which fills a gap defined between said semiconductor device and said
substrate in such a way that said semiconductor device and said substrate
are mechanically joined together.
2. A semiconductor unit package of claim 1, wherein:
said composition consists essentially of (A) a resin binder that contains
at least a polyepoxide, a carboxylic acid's anhydride, a rheology
modifier, and a latent curing accelerator, and (B) a filler that is formed
of a dielectric material; and
said rheology modifier functions to impede interaction between a free acid
in said anhydride of said carboxylic acid and a polar group at the surface
of said filler.
3. A semiconductor unit package of claim 2 wherein said rheology modifier
contains a substance capable of selective adsorption of said free acid in
said anhydride of said carboxylic acid.
4. A semiconductor unit package of claim 2 wherein said rheology modifier
is a Lewis-base compound.
5. A semiconductor unit package of claim 2 wherein said rheology modifier
is either a tertiary amine compound, a tertiary phosphine compound, a
quaternary ammonium salt, a quaternary phosphonium salt, or a heterocyclic
compound that contains in a cyclic chain thereof an atom of nitrogen.
6. A semiconductor unit package of claim 2 wherein said anhydride of said
carboxylic acid in said resin binder contains at least an anhydride of an
alicyclic acid.
7. A semiconductor unit package of claim 6 wherein said alicyclic acid
anhydride contains at least an anhydride of a trialkyltetrahydrophthalic
acid.
8. A semiconductor unit package of claim 1 wherein said bump electrode of
said semiconductor device is a stud bump electrode with a two stepped
protuberance.
9. A semiconductor unit packaging method wherein a semiconductor device
having an electrode pad is mounted on a substrate having a terminal
electrode, said method comprising:
(a) a first step of forming a bump electrode on said electrode pad of said
semiconductor device;
(b) a second step of applying a conductive adhesive around the tip of said
bump electrode;
(c) a third step comprising:
performing alignment of said bump electrode and said terminal electrode;
placing said semiconductor device onto said substrate; and
establishing, through said conductive adhesive, an electrical connection
between said bump electrode and said terminal electrode;
(d) a fourth step of preparing an encapsulant formed of a composition the
viscosity and thixotropy index of which are below 100 Pa.multidot.s and
below 1.1, respectively;
(e) a fifth step of filling a gap defined between said semiconductor device
and said substrate with said encapsulant; and
(f) a sixth step of curing said encapsulant to mechanically joint said
semiconductor device and said substrate.
10. A semiconductor unit packaging method of claim 9 wherein:
said composition of said fourth step consists essentially of (A) a resin
binder that contains at least a polyepoxide, an anhydride of a carboxylic
acid, a rheology modifier, and a latent curing accelerator, and (B) a
filler that is formed of a dielectric material; and
said rheology modifier functions to impede interaction between a free acid
in said carboxylic acid's anhydride and a polar group at the surface of
said filler.
11. A semiconductor unit packaging method of claim 9 wherein said rheology
modifier contains a substance, which acts also as a curing accelerator for
a double-liquid type encapsulant, by such a trace amount as to prevent
said substance from exhibiting its curing accelerant function.
12. A semiconductor unit packaging method of claim 10 wherein said
anhydride of said carboxylic acid in said resin binder of said fourth step
contains at least an anhydride of an alicyclic acid.
13. A semiconductor unit packaging method of claim 12 wherein said
alicyclic acid anhydride of said fourth step contains at least an
anhydride of a trialkyltetrahydrophthalic acid.
14. A semiconductor unit packaging method of claim 9 wherein said bump
electrode of said first step is a stud bump electrode with a two stepped
protuberance.
15. A semiconductor unit packaging method of claim 9 wherein in said fifth
step said encapsulant is injected between said semiconductor device and
said substrate at room temperature.
16. A semiconductor unit packaging method of claim 9 wherein in said fifth
step said encapsulant is injected between said semiconductor device and
said substrate under a depressurized condition.
17. A semiconductor unit packaging method of claim 9 wherein in said fourth
step said composition of said encapsulant is prepared by:
providing a mixture of an anhydride of a carboxylic acid and a part of a
filler;
subjecting said mixture to an aging process; and
adding a polyepoxide and the remaining filler to said mixture.
18. A semiconductor unit packaging method of claim 10 wherein said rheology
modifier contains a substance capable of selective adsorption of said free
acid in said anhydride of said carboxylic acid.
19. A semiconductor unit packaging method of claim 10 wherein said rheology
modifier is a Lewis-base compound.
20. A semiconductor unit packaging method of claim 10 wherein said rheology
modifier is either a tertiary amine compound, a tertiary phosphine
compound, a quaternary ammonium salt, a quaternary phosphonium salt, or a
heterocyclic compound that contains in a cyclic chain thereof an atom of
nitrogen.
21. An encapsulant for filling a gap between a semiconductor device and a
substrate for use in the packaging of a semiconductor unit, said
encapsulant essentially consisting of:
(A) a resin binder that contains at least a polyepoxide, an anhydride of a
carboxylic acid, a rheology modifier, and a latent curing accelerator
wherein the weight percentage of said resin binder is within the range of
from 80% to 25%; and
(B) a filler that is formed of a dielectric material wherein the weight
percentage of said filler is within the range of from 20% to 75%;
wherein said rheology modifier functions to impede interaction between a
free acid in said anhydride of said carboxylic acid and a polar group at
the surface of said filler.
22. An encapsulant of claim 21 wherein said rheology modifier contains a
substance capable of selective adsorption of said free acid in said
anhydride of said carboxylic acid.
23. An encapsulant of claim 21 wherein said rheology modifier is a
Lewis-base compound.
24. An encapsulant of claim 21 wherein said rheology modifier is either a
tertiary amine compound, a tertiary phosphine compound, a quaternary
ammonium salt, a quaternary phosphonium salt, or a heterocyclic compound
that contains in a cyclic chain thereof an atom of nitrogen.
25. An encapsulant of claim 21 wherein said anhydride of said carboxylic
acid in said resin binder contains at least an anhydride of an alicyclic
acid.
26. An encapsulant of claim 25 wherein said alicyclic acid anhydride
contains at least an anhydride of a trialkyltetrahydrophthalic acid.
27. An encapsulant of claim 21 wherein said resin binder and said filler
are arranged to stay as a single liquid.
28. An encapsulant of claim 21,
said resin binder having a composition wherein:
(a) the chemical equivalent ratio of said anhydride of said carboxylic acid
to said polyepoxide is within the range of from 0.8 to 1.1;
(b) the weight percentage of said curing accelerator to the entirety of
said resin binder is within the range of from 0.3% to 3%; and
(c) the weight percentage of said rheology modifier to the entirety of said
resin binder is within the range of from 0.02% to 0.3%.
29. An encapsulant for filling a gap between a semiconductor device and a
substrate for use in the packaging of a semiconductor unit, said
encapsulant essentially consisting of:
(A) a resin binder that contains at least a polyepoxide, an anhydride of a
carboxylic acid, a rheology modifier, and a latent curing accelerator
wherein the weight percentage of said resin binder is within the range of
from 80% to 25%; and
(B) a filler that is formed of a dielectric material wherein the weight
percentage of said filler is within the range of from 20% to 75%;
wherein said encapsulant is prepared by:
providing a mixture of an anhydride of a carboxylic acid and a part of a
filler;
subjecting said mixture to an aging process; and
adding a polyepoxide and the remaining filler to said mixture.
Description
BACKGROUND OF THE INVENTION
This invention is directed generally to a semiconductor unit package and to
a semiconductor unit packaging method. More specifically, the present
invention pertains to a technique wherein a semiconductor device is
mounted by means of flip-chip bonding onto a substrate, with a conductive
adhesive sandwiched therebetween, and the substrate and the semiconductor
device are mechanically connected together, with a resin encapsulating
layer sandwiched therebetween.
Generally, solder interconnection has been used for establishing
interconnections between connection terminals of electronic components
such as semiconductor devices and terminal electrodes of circuit patterns
on a substrate. As the size of semiconductor packages has been decreased
dramatically, and as the spacing between connection terminals has been
reduced owing to, for example, increase in the number of connection
terminals, conventional soldering finds it difficult to catch up with such
recent advances, since it requires large adhesive area.
Various flip-chip bonding approaches, in which the chip is flipped or
inverted such that its active element surface faces the substrate and is
directly connected to the substrate with terminal electrodes, have been
proposed for effective use of packaging areas. Typical examples of the
flip-chip bonding are described below.
(1) Junction By Low Melting-Point Metal
As shown in FIG. 8, a solder bump electrode 8 is formed on an electrode pad
2 of a semiconductor device 1. The solder bump electrode 8 is aligned with
a terminal electrode 5 on a substrate 6. Thereafter, solder is melted to
establish electrical connection between the semiconductor device 1 and the
substrate 6. FIG. 9 shows a technique similar to the one of FIG. 8. In
this technique, a bump electrode 3 of gold is formed. A deposit of a low
melting-point metal, e.g., a deposit 9 of indium, is formed between the
gold bump electrode 3 and the terminal electrode 5. The indium deposit 9
is melted and the bump electrode 3 and the terminal electrode 5 are
electrically connected together. Subsequently, the semiconductor device 1
and the substrate 6 are mechanically connected together, with an
encapsulating layer 10 sandwiched therebetween.
(2) Junction By Curing Contraction Stress
As shown in FIG. 10, a bump electrode 3 of gold is formed on an electrode
pad 2 of a semiconductor device 1. Alignment of the bump electrode 3 on
the semiconductor device 1 with a terminal electrode 5 on a substrate 6 is
carried out. Then, an encapsulating material is filled between the
semiconductor device 1 and the substrate 6. This encapsulating material
cures or hardens to form an encapsulating layer 12. Contraction stress
produced by such hardening results in application of compressive stress
between the bump electrode 3 and the terminal electrode 5, whereupon the
bump electrode 3 and the terminal electrode 5 are electrically connected
together and, at the same time, the semiconductor device 1 and the
substrate 6 are mechanically connected together. Additionally, in order to
improve connection reliability, a deposit 11 of gold may be formed on the
terminal electrode 5 (see FIG. 10).
(3) Junction By Anisotropic Conductive Adhesive
Referring now to FIG. 11, a bump electrode 3 of gold is formed on an
electrode pad 2 of a semiconductor device 1. An anisotropic conductive
adhesive, which includes a binder in which conductive particles are
dispersed, is filled between the semiconductor device 1 and a substrate 6.
This conductive adhesive is heated while at the same time having
application of pressure, whereupon it cures or hardens to form an
anisotropic conductive adhesive layer 13. As a result, the bump electrode
3 and a terminal electrode 5 are electrically connected together and, at
the same time, the semiconductor device 1 and the substrate 6 are
mechanically connected together.
(4) Junction By Conductive Adhesive
As shown in FIG. 12, a bump electrode 3 of gold is formed on an electrode
pad 2 of a semiconductor device 1. Thereafter, a conductive adhesive is
transferred to the bump electrode 3. Alignment of the bump electrode 3
with a terminal electrode 5 formed on a substrate 6 is carried out and
thereafter the transferred conductive adhesive cures to form a conductive
adhesive layer 4. As a result, the bump electrode 3 and the terminal
electrode 5 are electrically connected together, with the conductive
adhesive layer 4 sandwiched therebetween. An encapsulating material is
filled between the semiconductor device 1 and the substrate 6, as a result
of which the semiconductor device 1 and the substrate 6 are mechanically
connected together. This encapsulating material cures to form an
encapsulating layer 7, whereupon the semiconductor device 1 and the
substrate 6 are mechanically connected together. A typical encapsulating
material has a composition essentially formed of (a) a resin binder
including a cresol NOVOLAC type epoxy resin and a NOVOLAC type phenol
resin (curing agent) and (b) a filler formed of dielectric particles.
The above-described packaging techniques (1)-(4), however, have their
respective drawbacks.
The packaging techniques (1) and (2) have the problem that, since their
structures have difficulties in reducing thermal stress produced by the
difference in expansion coefficient between semiconductor device and
substrate, they are unsuitable for applications where connection stability
is required over a wide range of temperature.
Next, the packaging technique (3) is discussed. The packaging technique (3)
employs an anisotropic conductive adhesive that contains a resin binder
formed of a resin material with high flexibility, thereby making it
possible to reduce thermal stress. In spite of such an advantage, the
hygroscopic of the resin binder increases and the packaging technique (3)
suffers the problem of connection stability under conditions of high
humidity. Additionally, in the packaging technique (3). it is possible to
reduce thermal stress by matching the thermal expansion coefficient of the
binder with that of the semiconductor device 1 and with that of the
substrate 6. However, a filler having a low thermal expansion coefficient
is contained in large amounts, so that connection reliability at the early
stage is likely to be degraded.
Finally, the packaging technique (4) is discussed. This packaging technique
(4) is able to reduce thermal stress by a conductive adhesive with
flexibility and by matching the thermal expansion coefficient of the
encapsulating material with that of the semiconductor device 1 and with
that of the substrate 6. Because of such an advantage, the packaging
technique (4) appears to be most attractive as compared to the other
packaging techniques.
The packaging technique (4), however, has the following drawbacks. The
previously-described encapsulant, which is formed of a mixture composition
of (A) a cresol NOVOLAC type epoxy resin and (B) a NOVOLAC type phenol
resin, has a high viscosity coefficient. Additionally, matching of thermal
expansion coefficients requires a high proportion in content of a filler
in the encapsulant, resulting in increasing the viscosity of the
encapsulant. Therefore, at the time of filling such an encapsulant between
the semiconductor device and the substrate, it becomes necessary to heat
the encapsulant up to 70-80 degrees centigrade or more to reduce the
viscosity. This results in poor productivity. Further, at the time of the
encapsulant filling, conductive interconnections may be damaged by thermal
stress produced by the thermal expansion difference when temperature is
increased, thereby reducing connection reliability.
On the other hand, a resin binder as an encapsulant may be used which is
formed essentially of (A) a polyepoxide the viscosity of which is very low
at normal room temperature and (B) an acid anhydride. Note that
"polyepoxide" is a general term for epoxy resins and/or epoxy compounds.
However, if a large quantity of a filler is added to such a resin binder
for the purpose of reducing the thermal expansion coefficient, this will
hold the viscosity of the encapsulant low but increase the thixotropy
index. This produces the problem that the encapsulant is unable to enter
between the semiconductor device and the substrate, or the problem that,
even if the encapsulant manages to enter, such entrance is accompanied
with a great number of air bubbles. The presence of such air bubbles in
the encapsulant contributes to non-uniformity in, for example, thermal
expansion of the cured encapsulant. Connection reliability is reduced. For
this reason, it has been considered impractical to use a resin of
polyepoxide and acid anhydride as a binder.
SUMMARY OF THE INVENTION
Bearing in mind the above-described problems with the prior art techniques,
this invention was made. Accordingly a general object of the present
invention is to provide an improved semiconductor unit package and
associated packaging method capable of achieving high connection
reliability and high productivity. The inventors of the present invention
investigated the limitation of the characteristics of viscosity and
thixotropy index necessary for obtaining desirable encapsulating
characteristics of fillers. It is to be noted that "polyepoxide" is a
general term for epoxy resins and/or epoxy compounds.
The inventors of this invention found out the fact that the reason that
conventional materials are unsuitable for an encapsulant lies not only in
viscosity but also in thixotropy index (high thixotropy index). For
example, for the case of resin binders containing polyepoxides and acid
anhydrides, the inventors of the present invention found out that the
flowability is impeded by interaction between free acids in the acid
anhydride and polar groups at the surface of a filler. From this knowledge
found out by the present inventors, the following means are provided to
achieve the object of the present invention.
More specifically, in accordance with the present invention, a composition,
the viscosity and the thixotropy of which are below 100 Pa.multidot.s and
below 1.1, respectively, is used as an encapsulating material in the
flip-chip bonding. This composition cures to form an encapsulating layer
by which a semiconductor device and a substrate are mechanically connected
together.
The present invention provides a semiconductor unit package that comprises:
(a) a semiconductor device having an electrode pad;
(b) a substrate having a terminal electrode;
(c) a bump electrode formed on the electrode pad of the semiconductor
device;
(d) a conductive adhesive layer which is formed of a conductive adhesive
with flexibility and which establishes an electrical connection between
the bump electrode and the terminal electrode; and
(e) an encapsulating layer which is formed by curing a composition having a
viscosity of below 100 Pa.multidot.s and a thixotropy index of below 1.1
and which fills a gap defined between the semiconductor device and the
substrate in such a way that the semiconductor device and the substrate
are mechanically joined together.
The encapsulating layer, which mechanically joins a semiconductor device
and a substrate, is formed of an encapsulant, which is in the state of
liquid in a packaging step and which has not only a low viscosity
coefficient of below 100 Pa.multidot.s but also a low thixotropy index of
below 1.1. As a result of such arrangement, in a packaging step, such an
encapsulant readily flows and spreads, even into tiny gaps with no air
bubbles. The temperature of filling may be decreased. These arrangements
make it possible to improve not only electrical connection reliability
(e.g., semiconductor device-to-substrate adhesion, and resistance to
thermal shock) but also productivity.
It is preferred that the composition consists essentially of (A) a resin
binder that contains at least a polyepoxide, a carboxylic acid's
anhydride, a rheology modifier, and a latent curing accelerator, and (B) a
filler that is formed of a dielectric material, and that the rheology
modifier functions to impede interaction between a free acid in the
anhydride of the carboxylic acid and a polar group at the surface of the
filler.
It is preferred that the rheology modifier contains a substance capable of
selective adsorption of the free acid in the anhydride of the carboxylic
acid.
It is preferred that the rheology modifier is a Lewis-base compound.
It is preferred that the rheology modifier is either a tertiary amine
compound, a tertiary phosphine compound, a quaternary ammonium salt, a
quaternary phosphonium salt, or a heterocyclic compound that contains in a
cyclic chain thereof an atom of nitrogen.
As described above, the encapsulant is formed essentially of (A) an acid
anhydride-curing type epoxy resin and (B) a material having a low thermal
expansion coefficient (e.g., a dielectric material). This arrangement
reduces thermal stresses applied to the encapsulating layer. Additionally,
the rheology modifier used is a rheology modifier operable to impede
interaction between a free acid in the acid anhydride and a polar group at
the surface of the filler and hence a low viscosity coefficient and a low
thixotropy index can be achieved.
It is preferred that the anhydride of the carboxylic acid in the resin
binder contains at least an anhydride of an alicyclic acid.
It is preferred that the foregoing alicyclic acid anhydride contains at
least an anhydride of a trialkyltetrahydrophthalic acid.
The characteristics of alicyclic acid's anhydrides with low water
absorption are utilized to give the desirable resistance of resin binder
to moisture. Additionally, the viscosity of the resin binder which is in
the state of liquid in a packaging step is low, so that encapsulant
filling can be finished in a short time. The costs of production can be
cut down.
It is preferred that the bump electrode of the semiconductor device is a
stud bump electrode with a two stepped protuberance.
Such arrangement makes it possible to increase the density of bump
electrode. When mounting a semiconductor device onto a substrate,
densely-placed bump electrodes of the semiconductor device and terminal
electrodes on the substrate are electrically connected together.
Subsequently, an encapsulant having a low viscosity coefficient and a low
thixotropy index is employed so that it can readily flow and fill a gap
defined between the semiconductor device and the substrate. As a result,
even in high-density semiconductor units, electrical and mechanical
connections between semiconductor device and substrate are improved in
reliability.
The present invention provides a semiconductor unit packaging method
wherein a semiconductor device having an electrode pad is mounted on a
substrate having a terminal electrode. More specifically, this method
comprises:
(a) a first step of forming a bump electrode on the electrode pad of the
semiconductor device;
(b) a second step of applying a conductive adhesive around the tip of the
bump electrode;
(c) a third step comprising:
performing alignment of the bump electrode and the terminal electrode;
placing the semiconductor device onto the substrate; and
establishing, through the conductive adhesive, an electrical connection
between the bump electrode and the terminal electrode;
(d) a fourth step of preparing an encapsulant formed of a composition the
viscosity and thixotropy index of which are below 100 Pa.multidot.s and
below 1.1, respectively;
(e) a fifth step of filling a gap defined between the semiconductor device
and the substrate with the encapsulant; and
(f) a sixth step of curing the encapsulant to mechanically joint the
semiconductor device and the substrate.
Since the encapsulant has not only a low viscosity coefficient of below 100
Pa.multidot.s but also a low thixotropy index of below 1.1, this makes it
possible for such an encapsulant in a packaging step to readily flow and
spread, even into tiny gaps with no air bubbles. The temperature of
filling may be decreased. These arrangements make it possible to improve
not only electrical connection reliability (e.g., semiconductor
device-to-substrate adhesion, and resistance to thermal shock) but also
productivity, and to shorten the packaging time.
It is preferred that the composition of the fourth step consists
essentially of (A) a resin binder that contains at least a polyepoxide, an
anhydride of a carboxylic acid, a rheology modifier, and a latent curing
accelerator, and (B) a filler that is formed of a dielectric material, and
that the rheology modifier functions to impede interaction between a free
acid in the carboxylic acid's anhydride and a polar group at the surface
of the filler.
Such arrangement makes it possible to reduce both the viscosity and
thixotropy index of the encapsulant in the fifth step. Additionally, the
encapsulant is formed essentially of (A) an acid anhydride-curing type
epoxy resin and (B) a material having a low thermal expansion coefficient
(e.g., a dielectric material). This arrangement reduces thermal stresses
applied after the packaging to the encapsulating layer.
It is preferred that the rheology modifier contains a substance, which acts
also as a curing accelerator for a double-liquid epoxy resin type
encapsulant, by such a trace amount as to prevent the substance from
exhibiting its curing function.
Such arrangement controls an encapsulant in such a way that the encapsulant
does not start curing between the fourth step and the fifth step and is
cured in the sixth step. When cured in the sixth step, a rheology modifier
is incorporated into an encapsulating resin layer's network structure.
This eliminates the possibility that addition of a rheology modifier
reduces resistance to heat and resistance to moisture.
It is preferred that the anhydride of the carboxylic acid in the resin
binder of the fourth step contains at least an anhydride of an alicyclic
acid.
It is preferred that the alicyclic acid anhydride of the fourth step
contains at least an anhydride of a trialkyltetrahydrophthalic acid.
Since an anhydride of an alicyclic acid is low in viscosity as well as in
water absorption, time required for filling of an encapsulant in the sixth
step is reduced, and resistance to moisture is enhanced.
It is preferred that the bump electrode of the first step is a stud bump
electrode with a two stepped protuberance.
Such arrangement enables the high-density placement of bump electrodes, and
the encapsulant, which is low in viscosity as well as in thixotropy index,
readily spreads, even into tiny gaps defined between the densely-placed
bump electrodes and terminal electrodes of the substrates. As a result,
electrical and mechanical connections between semiconductor device and
substrate are improved in reliability.
It is preferred that in the fifth step the encapsulant is injected between
the semiconductor device and the substrate at room temperature.
Such arrangement achieves a reduction of the thermal stress thereby
improving resistance to thermal shock. As a result, a semiconductor unit
package with improved electrical connection reliability is achieved.
It is preferred that in the fifth step the encapsulant is injected between
the semiconductor device and the substrate under a depressurized
condition.
Such arrangement not only achieves an improvement in productivity but also
provides a semiconductor unit package with improved electrical connection
reliability.
It is preferred that in the fourth step the composition of the encapsulant
is prepared by providing a mixture of an anhydride of a carboxylic acid
and a part of a filler, subjecting the mixture to an aging process, and
adding a polyepoxide and the remaining filler to the mixture.
As a result of such arrangement, interaction between a free acid and a
polar group is diminished. This achieves an encapsulant having a low
viscosity and a low thixotropy index.
It is preferred that the rheology modifier contains a substance capable of
selective adsorption of the free acid in the anhydride of the carboxylic
acid.
As a result of such arrangement, a free acid in an acid anhydride is
selectively adsorbed by a rheology modifier, whereupon interaction between
free acid and polar group is impeded. This achieves an encapsulant having
a low viscosity and a low thixotropy index.
It is preferred that the rheology modifier is a Lewis-base compound.
It is preferred that the rheology modifier is either a tertiary amine
compound, a tertiary phosphine compound, a quaternary ammonium salt, a
quaternary phosphonium salt, or a heterocyclic compound that contains in a
cyclic chain thereof an atom of nitrogen.
As a result of such arrangements, interaction between free acid and polar
group is impeded. This achieves an encapsulant having a low viscosity and
a low thixotropy index.
The present invention provides an encapsulant for filling a gap between a
semiconductor device and a substrate for use in the packaging of a
semiconductor unit. This encapsulant essentially consists of:
(A) a resin binder that contains at least a polyepoxide, an anhydride of a
carboxylic acid, a rheology modifier, and a latent curing accelerator
wherein the weight percentage of the resin binder is within the range of
from 80% to 25%; and
(B) a filler that is formed of a dielectric material wherein the weight
percentage of the filler is within the range of from 20% to 75%;
wherein the rheology modifier functions to impede interaction between a
free acid in the anhydride of the carboxylic acid and a polar group at the
surface of the filler.
Since the encapsulant has not only a low viscosity coefficient of below 100
Pa.multidot.s but also a low thixotropy index of below 1.1, this makes it
possible for such an encapsulant to readily flow and spread, even into
tiny gaps with no air bubbles. The temperature of filling may be lowered.
Additionally, the latent curing accelerator ensures the storage stability
of the encapsulant and practical curing accelerant function. These
arrangements make it possible to improve not only electrical connection
reliability (e.g., semiconductor device-to-substrate adhesion, and
resistance to thermal shock) but also productivity.
It is preferred that the rheology modifier contains a substance capable of
selective adsorption of the free acid in the anhydride of the carboxylic
acid.
It is preferred that the rheology modifier is a Lewis-base compound.
It is preferred that the rheology modifier is either a tertiary amine
compound, a tertiary phosphine compound, a quaternary ammonium salt, a
quaternary phosphonium salt, or a heterocyclic compound that contains in a
cyclic chain thereof an atom of nitrogen.
The encapsulant is formed essentially of (A) an acid anhydride-curing type
epoxy resin and (B) a material having a low thermal expansion coefficient
(e.g., a dielectric material). This arrangement reduces thermal stresses
applied to the encapsulating layer in a semiconductor unit package to be
formed. Additionally, the rheology modifier used is a rheology modifier
operable to impede interaction between a free acid in the acid anhydride
and a polar group at the surface of the filler and hence a low viscosity
coefficient and a low thixotropy index can be achieved.
It is preferred that the anhydride of the carboxylic acid in the resin
binder contains at least an anhydride of an alicyclic acid.
It is preferred that the aforesaid alicyclic acid anhydride contains at
least an anhydride of a trialkyltetrahydrophthalic acid.
The characteristics of alicyclic acid's anhydrides with low water
absorption are utilized to give the desirable resistance of resin binder
to moisture. Additionally, the viscosity of the resin binder which is in
the state of liquid in a packaging step is low, so that encapsulant
filling can be finished in a short time. The costs of packaging can be cut
down.
It is preferred that the resin binder and the filler are arranged to stay
as a single liquid.
Such arrangement facilitates uniform dispersion of the filler thereby
providing a desirable encapsulant for the manufacture of LSIs.
It is preferred that the resin binder in the encapsulant has a composition
wherein:
(a) the chemical equivalent ratio of the anhydride of the carboxylic acid
to the polyepoxide is within the range of from 0.8 to 1.1;
(b) the weight percentage of the curing accelerator to the entirety of the
resin binder is within the range of from 0.3% to 3%; and
(c) the weight percentage of the rheology modifier to the entirety of the
resin binder is within the range of from 0.02% to 0.3%.
The present invention provides an encapsulant for filling a gap between a
semiconductor device and a substrate for use in the packaging of a
semiconductor unit. This encapsulant essentially consists of:
(A) a resin binder that contains at least a polyepoxide, an anhydride of a
carboxylic acid, a rheology modifier, and a latent curing accelerator
wherein the weight percentage of the resin binder is within the range of
from 80% to 25%; and
(B) a filler that is formed of a dielectric material wherein the weight
percentage of the filler is within the range of from 20% to 75%;
wherein the encapsulant is prepared by:
providing a mixture of an anhydride of a carboxylic acid and a part of a
filler;
subjecting the mixture to an aging process; and
adding a polyepoxide and the remaining filler to the mixture.
As a result of such arrangement, interaction between the free acid in the
anhydride of the carboxylic acid and the polar group at the surface of the
filler, is suppressed and the thixotropy index of the encapsulant is
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of a semiconductor unit of an embodiment in
accordance with the present invention.
FIG. 2 is a cross-section of a joint of the FIG. 1 semiconductor unit.
FIG. 3 is a cross-section of a semiconductor unit formed by a stud bump
technique of an embodiment in accordance with the present invention.
FIGS. 4(a)-4(e) are cross-sections of a semiconductor unit at different
process stages of a flip-chip bonding technique of an embodiment in
accordance with the present invention.
FIG. 5 is a flow diagram showing steps of a flip-chip bonding technique of
an embodiment in accordance with the present invention.
FIG. 6 shows the generic chemical composition of a bisphenol type epoxy
resin in a resin binder used in an embodiment in accordance with the
present invention.
FIG. 7 shows the generic chemical composition of a
trialkyltetrahydrophthalic acid in a resin binder used in an embodiment in
accordance with the present invention.
FIG. 8 is a cross section of a conventional semiconductor unit in which
connection is established by a solder bump electrode.
FIG. 9 is a cross section of a conventional semiconductor unit in which
connection is established by a low melting-point metal layer.
FIG. 10 is a cross section of a conventional semiconductor unit in which
connection is established by making use of contraction stresses exerted
when an encapsulating resin cures.
FIG. 11 is a cross section of a conventional semiconductor unit in which
connection is established by an anisotropic conductive adhesive.
FIG. 12 is a cross section of a conventional semiconductor unit in which
connection is established by a conductive adhesive.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention are described by making
reference to the accompanying drawing figures.
FIG. 1 is a cross section depicting a semiconductor unit package in
accordance with the present invention. FIG. 2 is a cross section of a
joint of the FIG. 1 semiconductor unit package. This semiconductor unit
package is formed by a flip-chip bonding method. Reference numeral 1
denotes a semiconductor device such as an LSI chip. Reference numeral 2
denotes an electrode pad formed in the semiconductor device 1. Reference
numeral 3 denotes a bump electrode of gold. Reference numeral 4 denotes a
conductive adhesive layer of a composition (i.e., a conductive adhesive)
essentially formed of a special epoxy resin and conductive powders of, for
example, an alloy of AgPd. Reference numeral 6 is a substrate, e.g., a
ceramic substrate, onto which the semiconductor device 1 is mounted.
Reference numeral 5 denotes a terminal electrode formed on the substrate
6. Reference numeral 7 denotes an encapsulating layer formed of an
encapsulant. Such an encapsulant is essentially formed of an acid
anhydride-curing type epoxy resin. This encapsulant 7, when it remains
fluid, has a thixotropy index of below 1.1 and a viscosity coefficient of
100 Pa.multidot.s. The encapsulant 7 is injected between the semiconductor
device 1 and the substrate 6 by capillary action and is cured. It is to be
noted that the thixotropy index is the index expressed by
.DELTA..eta./.DELTA..epsilon. where .epsilon. is the shear rate and .eta.
is the viscosity coefficient. Here, the thixotropy index when the shear
rate .epsilon. falls in the range of 2 (1/sec) to 20 (1/sec), is shown.
FIG. 3 is a cross-section of a semiconductor unit package by means of a
flip-chip bonding method using a stud bump electrode. The semiconductor
unit package of FIG. 3 and the semiconductor unit package of FIG. 1 are
basically the same, except that the former semiconductor unit package
employs a stud bump electrode 14 with a two stepped protuberance instead
of the bump electrode 13. Employment of a flip-chip bonding method using a
stud bump electrode with a two stepped protuberance makes it possible to
deal with a semiconductor device with a greater number of electrode pads,
which is detailed later.
A flip-chip bonding method, which uses the stud bump electrode 14 of FIG.
3, is illustrated by making reference to FIGS. 4(a)-4(e) and to FIG. 5.
FIGS. 4(a)-4(e) are cross-sections of a semiconductor unit package at
different stages of a flip-chip bonding method. FIG. 5 is a flow diagram
showing steps of the flip-chip bonding method. The packaging process is
described step by step with reference to FIG. 5.
At step ST1, an wire of gold (Au) is used to form stud bump electrodes 14
at electrode pads 2 in the semiconductor device (LSI chip) 1. At step ST2,
a leveling process is carried out and each stud bump electrode 14 is
pressed against a level surface so that the leading ends of the stud bump
electrodes 14 are flush with one another.
Next, at step ST3, as shown in FIGS. 4(a)-4(c), the semiconductor device 1,
with the side of the stud bump electrode 14 facing down, is placed above a
substrate 20 with application of a conductive adhesive 4a. Thereafter, the
semiconductor device 1 is lowered towards the substrate 20 such that the
stud bump electrode 14 is soaked in the conductive adhesive 4a on the
substrate 20. Subsequently, the semiconductor device 1 is lifted up, as a
result of which transfer of the conductive adhesive 4a onto the stud bump
electrode 14 is completed.
Next, at steps ST4 and ST5, as shown in FIG. 4(d), the semiconductor device
1 is placed onto the ceramic substrate 6 having thereon the terminal
electrode 5. At this time, alignment of the stud bump electrode 14 of the
semiconductor device 1 with the terminal electrode 5 of the substrate 6 is
carried out, and the conductive adhesive 4a is heated to cure to form the
conductive adhesive layer 4. As a result, the stud bump electrode 14 of
the semiconductor device 1 and the terminal electrode 5 of the substrate 6
are electrically connected together.
At step ST6, testing for the presence or absence of an electrical
connection failure is carried out. If an electrical connection failure is
found, chip replacement is carried out at step ST7 and the flip-chip
bonding process returns back to step ST4. If no electrical connection
failure is found, the process advances to step ST8.
At step ST8, an encapsulant, which is formed of a composition having a low
viscosity of below 100 Pa.multidot.s and a low thixotropy index of below
1.1, is injected between the semiconductor device 1 and the substrate 6 at
normal room temperature to resin-encapsulate connection parts.
Subsequently, at step ST9, a heating treatment is carried out to cure a
resin binder contained in the injected encapsulant. As a result, the
encapsulating layer 7 is formed (see FIG. 4(e)), whereupon the
semiconductor device 1 and the substrate 6 are mechanically connected
together by the encapsulating layer 7.
At step ST10, final testing is made and the flip-chip bonding process is
completed.
The present embodiment employs a low-viscosity, low thixotropy-index
encapsulant. This produces the advantages that an encapsulant injection
process can be done smoothly even at room temperature and the encapsulant
injected readily flows and well spreads to fill a tiny gap between the
semiconductor device 1 and the substrate 6. This is time saving, and the
connection reliability of a joint made by the conductive adhesive 4 can be
maintained. Additionally, the encapsulant is a composition essentially
formed of (a) an acid anhydride-curing type epoxy resin with improved
flowability and (b) a filler such a fused silica, in other words it has a
low post-curing thermal expansion coefficient. Since the coefficient of
thermal expansion of the encapsulating layer 7 is low, this controls
thermal stresses produced by the differences in the coefficient of thermal
expansion between the semiconductor device 1 of silicon and the substrate
6 of, for example, alumina. Additionally, an encapsulant, formed of a
resin of the epoxy group, exhibits high resistance to heat and has strong
adhesion, therefore achieving connection reliability that remains stable
even under high temperature and high humidity conditions.
Since the conductive adhesive 4 has great flexibility, this contributes to
reducing thermal stresses and connection reliability is further improved.
In the present embodiment, the bump electrode 3 is formed of gold. Other
functionally equivalent metals, e.g., copper, may be used to form the bump
electrode 3. Additionally, in the present embodiment, the stud bump
electrode 14 is used. Other types of bump electrodes used in usual
flip-chip bonding techniques may be used. It is, however, to be noted that
use of stud bump electrodes controls lateral spreading of the conductive
adhesive layer 4 thereby achieving a much higher packaging density.
In the present embodiment, the conductive adhesive 4 is formed of a
material of the epoxy group. Other materials with flexibility may be used,
e.g., a material of the rubber group (e.g., SBR, NBR, IR, BR, CR), a
material of the acrylic group, a material of the polyester group, a
material of the polyamide group, a material of the polyether group, a
material of the polyurethane group, a material of the polyimide group, and
a material of the silicon group. As a conductive powder material that is
contained in the conductive adhesive, powders of noble metals (silver,
gold, palladium), powders of base metals (nickel, copper), powders of
alloys (solder, AgPd), mixture powders of silver-plated copper, and
powders of nonmetals with conductivity (carbon). These powders may be used
separately or in combination. The diameter of powders is not limited to a
particular one. The shape of powders is not limited to a particular one.
The encapsulant is formed essentially of (A) a resin binder and (B) a
filler. The resin binder's essential components are a polyepoxide, an acid
anhydride, and a rheology modifier. Such a polyepoxide is the so-called
epoxy compounds (epoxy resins) and there are no limitations on its
elements. Examples of the polyepoxide are a bisphenol type epoxy resin
(see the FIG. 6), a NOVOLAC type epoxy resin, a glycidylether type epoxy
resin, a glycidylester type epoxy resin, a glycidylamine type epoxy resin,
an alicyclic type epoxy resin, a biphenyl type epoxy resin, a naphthalene
type epoxy resin, a styrene oxide, an alkylglycidylether, and an
alkylglycidylester. They are used separately or in combination.
As the acid anhydride used here in the present invention, curing agents for
epoxy compounds and epoxy resins may be used. One of the most preferable
acid anhydrides is a trialkyltetrahydrophthalic acid's anhydride (see FIG.
7). Other preferable ones are a methyltetrahydrophthalic acid's anhydride,
and a methylhexahydrophthalic acid's anhydride and a methylhymic acid's
anhydride of the cyclic aliphatic group that are in the state of liquid at
25 degrees centigrade. Other acid anhydrides may be used. These acid
anhydrides may be used separately or in combination. If these acid
anhydrides mentioned above are used as predominant elements of the resin
binder, this provides an improved encapsulant that has very low viscosity,
high heat resistance, high humidity resistance, and high adhesion.
In addition to the foregoing essential elements of the resin binder, a
third binder element may be added as required, for improvement in heat
resistance, humidity resistance, adhesion strength and for adjustment of
thermal expansion coefficient, rheology, and reactivity.
Any powdery filler may be used as one of the predominant elements of the
encapsulant as long as its average particle diameter falls in the range of
from 1 .mu.m to 50 .mu.m. For example, silica oxides, alumina oxides,
aluminum nitrides, silicon carbides, and silicification compounds all of
which are thermally stable and have low thermal expansion coefficients.
These filler elements are used in any combination. There are no particular
limits on the filler dose, preferably 20-80 percent, on a weight basis, of
the entirety of the encapsulant. Use of these filler elements achieves an
improved encapsulant which is superior in insulation and which produces
less thermal stress.
Any rheology modifier for modification of the encapsulant flowability may
be used as long as it functions to prevent a free acid in the acid
anhydride from interacting with a polar group at the surface of the filler
and to reduce the thixotropy index of the encapsulant. The following are
preferred examples of the rheology modification.
(1) Rheology Modification Method I
In Method 1, an acid anhydride is pre-mixed with a part of a filler. The
mixture is subjected to an aging process. For example, the mixture may be
heated up to 100 degrees centigrade or less. This is followed by addition
of a polyepoxide compound, the remaining filler, and other addition
agents, to obtain a desirable encapsulant.
(2) Rheology Modification Method II
In Method II, a substance capable of selective adsorption of free acids in
an acid anhydride is added to an encapsulant.
(3) Rheology Modification Method III
In Method III, a substance (e.g., a Lewis-base compound having neither N--H
groups nor O--H groups) that interacts more strongly with a free acid than
a polar group at the surface of a filler, is added to an encapsulant.
Suitable Lewis-base compounds include tertiary amine compounds, tertiary
phosphine compounds, quaternary ammonium salts such as the
tetrabutylammonium bromide, quaternary phosphonium salts such as the
tetrabutylphosphoniumbenzotriazolate, melamines, and heterocyclic
compounds that contain in cyclic chains thereof atoms of nitrogen such as
imidazole compounds. There are many Lewis-base compounds other than the
above. These Lewis-base compounds may be used separately or in
combination.
The encapsulant may contain, as required, a solvent, a dispersing agent, a
rheology regulatory agent such as a leveling agent, an adhesion improving
agent such as a coupling agent, or a reaction regulatory agent such as a
curing accelerator.
The rheology modifier of the present invention, which consists of a
Lewis-base compound such as the amine compound, is usually used as a
reaction (curing) accelerator between a polyepoxide and an anhydride of an
carboxylic acid.
When the rheology modifier is used as a curing accelerator for an
encapsulant, curing reaction progresses even when stored at low
temperature to enter the stage of gel. This limits the type of encapsulant
to double-liquid type ones, in other words mixing must be made just before
use. On the other hand, LSI encapsulant require that large amounts of
fillers must be dispersed uniformly, in other words a single-liquid type
encapsulant is required for LSI.
To sum up, the rheology modifier of the present invention may be used as a
curing accelerator for a double-liquid type encapsulant but not for a
single-liquid type encapsulant.
If the dose is reduced to such an extent as to prevent gelling during the
storage, the present rheology modifier may find applications in the
single-liquid type encapsulant. In such a case, a curing accelerant
function that the rheology modifier performs is too poor to meet practical
requirements, in other words no high-level encapsulant curing
characteristics are obtained.
The present invention is characterized in that it employs a latent curing
accelerator with both storage stability and practical curing accelerant
functions, and that substances, e.g., amines, which are usually used as a
curing accelerator for the double-liquid type encapsulant, are employed as
a rheology modifier. Such a rheology modifier is added in such an amount
that it performs no curing functions but functions to improve interface
characteristics.
A latent curing accelerator is the catalyst whose catalyst activities are
greatly promoted upon application of, for example, thermal energy.
Generally, latent curing accelerators are melted (liquefied) or
reaction-dissociated upon application of energy, to be enhanced in
activity.
It is preferred that the encapsulant has the following composition.
______________________________________
Wt. percent
______________________________________
Resin binder 80-25
Filler element 20-75
______________________________________
It is preferred that the resin binder essentially consists of a
polyepoxide, an anhydride of a carboxylic acid, a curing accelerator, and
a rheology modifier according to the following element ratios.
______________________________________
Equivalent ratio
Carboxylic acid's anhydride/Polyepoxide
0.8-1.1
Wt. percent
Curing accelerator/Resin binder
0.3-3
Rheology modifier/Resin binder
0.02-0.3
______________________________________
In the present invention, the substrate 6 is formed of ceramic (e.g.,
alumina). Metal glaze substrates, glass substrates, resin substrates
(e.g., glass epoxy substrates), polymer film substrates are applicable.
There are no specific limitations on the material of the terminal electrode
5.
The following are embodiments for investigation of the characteristics of
semiconductor units obtained by the above-described flip-chip bonding
method.
EMBODIMENT 1
A semiconductor unit with the FIG. 1 structure is formed in accordance with
steps of FIGS. 4(a)-4(e). Bump electrode 3 is formed by means of gold
plating. Conductive adhesive 4a has a composition formed essentially of
powders of AgPd and an epoxy resin with flexibility. Conductive adhesive
4a is heated at 120 degrees centigrade and, as a result, cures. Further,
an encapsulant of COMPOSITION a in TABLE 1 is cured at 150 degrees
centigrade.
EMBODIMENT 2
Stud bump electrode 14 of FIG. 3 is formed on electrode pad 2 of
semiconductor device 1 by means of a gold-wire bonder. The following steps
are the same as the first embodiment and are carried out under the same
conditions as the first embodiment.
EMBODIMENT 3
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the first embodiment, except that in the third embodiment an
encapsulant injection process is carried out under depressurized
condition.
EMBODIMENT 4
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the fourth embodiment
COMPOSITION b of TABLE 1 is used.
EMBODIMENT 5
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the fifth embodiment
substrate 6 is a glass epoxy substrate and COMPOSITION c of TABLE 1 is
used.
EMBODIMENT 6
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the sixth embodiment
substrate 6 is a glass epoxy substrate, conductive adhesive 4 contains
powders of silver as conductive powders, and COMPOSITION d of TABLE 1 is
used.
EMBODIMENT 7
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the seventh embodiment
substrate 6 is a glass substrate, conductive adhesive 4 is formed
essentially of powders of silver and an urethane resin, COMPOSITION e of
TABLE 1 is used, and encapsulant injection is carried out under
depressurized condition.
EMBODIMENT 8
Bump electrode 3 of FIG. 1 is formed on electrode pad 2 of semiconductor
device 1 by means of gold plating. Semiconductor device 1 is mounted onto
substrate 6 in the same way as the seventh embodiment and under the same
conditions as the seventh embodiment.
COMPARE EXAMPLE 1
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the first compare
example COMPOSITION f of TABLE 1 is used.
COMPARE EXAMPLE 2
Semiconductor device 1 is mounted onto substrate 6 under the same
conditions as the second embodiment, except that in the second compare
example COMPOSITION g of TABLE 1 is used.
COMPOSITIONS a-g are shown below.
TABLE 1
______________________________________
COMPOSITION a:
bisphenol F type epoxy resin (epoxy equivalent: 162)
85 phr
bisphenol A type epoxy resin (epoxy equivalent: 182)
15 phr
trialkyltetrahydro phthalic acid's-anhydride
126 phr
(anhydride equiv.: 234)
2-(2-methylimidazolylethyl)-4, 6-diamino
1.6 phr
triazine-isocyanuric acid addition product
diazabicycloundecene 0.1 phr
fused silica 340 phr
COMPOSITION b:
trialkyltetrahydro phthalic acid's anhydride
126 phr
(anhydride equiv.: 234)
fused silica 340 phr
These two material were kneaded and subjected to
an aging process for 10 hours at 60 degrees
centigrade. Thereafter the following materials were
added to them.
bisphenol F type epoxy resin (epoxy equivalent: 162)
85 phr
bisphenol A type epoxy resin (epoxy equivalent: 182)
15 phr
2-(2-methylimidazolylethyl)-4, 6-diamino
1.6 phr
triazine-isocyanuric acid addition product
1-cyanoethyl-2-ethyl-4-methylimidazole
0.2 phr
COMPOSITION c:
bisphenol F type epoxy resin (epoxy equivalent: 162)
80 phr
alicyclic epoxy resin (ERL4221)*
20 phr
trialkyltetrahydro phthalic acid's anhydride
135 phr
(anhydride equiv.: 234)
AMICURE PN** 5 phr
tetrabutylammonium bromide 0.2 phr
fused silica 400 phr
COMPOSITION d:
bisphenol F type epoxy resin (epoxy equivalent: 162)
90 phr
bisphenol A type epoxy resin (epoxy equivalent: 182)
10 phr
trialkyltetrahydro phthalic acid's anhydride
128 phr
(anhydride equiv.: 234)
FUJIHARD FXE1000*** 5 phr
tetrabutylphosphoniumbenzotriazolate
0.2 phr
fused silica 350 phr
COMPOSITION e:
bisphenol F type epoxy resin (epoxy equivalent: 162)
70 phr
naphthalene type epoxy resin (epoxy equivalent: 148)
30 phr
trialkyltetrahydro phthalic acid's anhydride
82 phr
(anhydride equiv.: 234)
methyltetrahydro phthalic acid's anhydride
40 phr
(anhydride equiv.: 166)
triphenylphosphinetriphenylborate
3.6 phr
tetrabutylphosphoniumbenzotriazolate
0.2 phr
fused silica 225 phr
COMPOSITION f:
bisphenol F type epoxy resin (epoxy equivalent: 162)
85 phr
bisphenol A type epoxy resin (epoxy equivalent: 182)
15 phr
trialkyltetrahydro phthalic acid's anhydride
126 phr
(anhydride equiv.: 234)
2-(2-methylimidazolylethyl)-4, 6-diamino
1.6 phr
triazine-isocyanuric acid addition product
fused silica 340 phr
COMPOSITION g:
bisphenol F type epoxy resin (epoxy equivalent: 162)
100 phr
alkyl modified phenol resin
70 phr
(hydroxyl group equiv: 113)
triphenylphosphine 0.6 phr
fused silica 255 phr
______________________________________
Note:
*= product by UCC;
**= product by AJINOMOTO; and
***= product by FUJI KASEI
COMPARE EXAMPLE 3
Semiconductor device 1 is mounted onto substrate 6 in a conventional way
shown in FIG. 9. Substrate 6 is an alumina substrate. Bump electrode 3 is
formed of gold. Terminal electrode 5 is indium-plated. Alignment of bump
electrode 3 with terminal electrode 5 is carried out and, thereafter
semiconductor device 1 is pressed by a jig and, at the same time, is
heated up to 170 degrees centigrade, whereupon bump electrode 3 and
terminal electrode 5 is connected together. Further, a silicon encapsulant
of zero stress type is injected between semiconductor device 1 and
substrate 6. This encapsulant is cured to form encapsulating layer 10.
COMPARE EXAMPLE 4
Semiconductor device 1 is mounted onto substrate 6 in a conventional way
shown in FIG. 10. Bump electrode 3 is formed of gold. Gold deposit 11 is
formed on terminal electrode 5. Gold deposit 11 is coated with an acrylic
encapsulant. Alignment of bump electrode 3 with terminal electrode 5 is
carried out. Subsequently, semiconductor device 1 is pressed by a jig
while at the same time the encapsulant is cured by UV irradiation or by
application of heat, to form encapsulating layer 12.
COMPARE EXAMPLE 5
Semiconductor device 1 is mounted onto substrate 6 in a conventional way
shown in FIG. 11. Bump electrode 3 is formed of gold. Substrate 6 is
formed of alumina. Alumina substrate 6 is coated with an anisotropic
conductive adhesive in which particles of gold are dispersed in an epoxy
binder. Alignment of bump electrode 3 and terminal electrode 5 is carried
out. Thereafter, semiconductor device 1 is pressed by a jig while at the
same time the adhesive is cured by UV irradiation or by application of
heat, to form anisotropic conductive adhesive layer 13. As a result, bump
electrode 3 and terminal electrode 5 are electrically and mechanically
connected together.
The viscosity, thixotropy index, injection time of each of the encapsulant
used in the first to eight embodiments and the first to fifth compare
examples, are shown below (TABLE 2).
TABLE 2
______________________________________
THIXOTROPY TIME
COMP. VISCOSITY INDEX (min)
______________________________________
EX. 1 & a 7 Pa .multidot. s
1.0 3.5
EX. 2
EX. 3 a 7 Pa .multidot. s
1.0 0.4
EX. 4 b 8 Pa .multidot. s
0.9 3.0
EX. 5 c 4 Pa .multidot. s
1.0 2.3
EX. 6 d 5 Pa .multidot. s
1.0 2.5
EX. 7 & e 11 Pa .multidot. s
1.0 0.6
EX. 8
COMPARE f 7 Pa .multidot. s
4.8 100 or
EX. 1 more
COMPARE g 120 Pa .multidot. s
1.3 45
EX. 2
______________________________________
NOTE:
VISCOSITY: measured by Etype viscometer (25.degree. C.; 10 rpm);
THIXOTROPY INDEX: measured by Etype viscometer (25.degree. C.; 1 rpm/10
rpm);
INJECTION TIME: time required for encapsulation of 5 mm square
semiconductor chip at 25.degree. C.
As can be seen from TABLE 2, in the embodiments of the present invention,
the injection time is short falling in the range of 0.4 to 3.5 minutes.
The present invention is suitable for practical applications, accordingly.
Conversely, in the compare examples, the injection time is much longer
than that of the present invention. The compare examples are unsuitable
for practical applications. TABLE 2 shows that the injection time
correlates with the viscosity/thixotropy index. In other words, in the
present invention, the viscosity is low (i.e., below 100 Pa.multidot.s)
and the thixotropy index is also low (i.e., below 1.1), which results in
reducing the encapsulant injection time. On the other hand, in the second
compare example, the viscosity exceeds 100 Pa.multidot.s and, in the first
compare example, the thixotropy index exceeds 1.1, which results in
greatly increasing the encapsulant injection time. To sum up, when the
encapsulant viscosity is below 100 Pa.multidot.s and when the thixotropy
index is below 1.1, the flowability of encapsulant becomes improved to be
suitable for practical applications.
For the purpose of evaluating the stability of connection in each of the
first to eighth embodiments of the present invention and the first to
fifth compare examples, various environmental tests were made as shown in
TABLES 3 and 4.
TABLE 3
______________________________________
TEST 1 TEST 2 TEST 3 TEST 4 TEST 5
______________________________________
EX. 1 .smallcircle.
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EX. 2 .smallcircle.
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.smallcircle.
EX. 3 .smallcircle.
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.smallcircle.
.smallcircle.
.smallcircle.
EX. 4 .smallcircle.
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.smallcircle.
.smallcircle.
EX. 5 .smallcircle.
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.smallcircle.
.smallcircle.
.smallcircle.
EX. 6 .smallcircle.
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EX. 7 .smallcircle.
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.smallcircle.
.smallcircle.
EX. 8 .smallcircle.
.smallcircle.
.smallcircle.
.smallcircle.
.smallcircle.
COMP. 1
.smallcircle.
.smallcircle.
x .smallcircle.
x
COMP. 2
.smallcircle.
.smallcircle.
x .smallcircle.
.smallcircle.
COMP. 3
.smallcircle.
.smallcircle.
x .smallcircle.
.smallcircle.
COMP. 4
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x x .smallcircle.
COMP. 5
x .smallcircle.
.smallcircle.
x x
______________________________________
Note:
TEST 1: semiconductor devices are subjected to a hightemperature conditio
for a specified period;
TEST 2: semiconductor devices are subjected to a lowtemperature condition
for a specified period;
TEST 3: semiconductor devices are subjected to a thermal shock;
TEST 4: semiconductor devices are subjected to a high humidity condition
for a specified period;
TEST 5: solderheat resistance testing.
TABLE 4
______________________________________
POST-TESTING
CONNECTION RESISTIVITY
BELOW 200 .OMEGA. = .smallcircle.
CRITERION ABOVE 200 .OMEGA. = x
______________________________________
TESTING CONDITIONS
TEST 1 150.degree. C.; 1000 hr
TEST 2 -55.degree. C.; 1000 hr
TEST 3 150 to -55.degree. C.; 500 cycles
TEST 4 121.degree. C.; 100%; 100 hr
TEST 5 270.degree. C.; 10 sec; 5 cycles
______________________________________
The evaluation results are demonstrated. As can be seen from the tables,
none of the first to eighth examples of this invention suffer problems in
connection stability. Each embodiment uses an encapsulant the viscosity
and thixotropy index of which are below 100 Pa.multidot.s and below 1.1,
respectively. Use of such a low-viscosity, low-thixotropy encapsulant
achieves semiconductor unit packages of high productivity and high
resistivity against various environmental conditions, regardless of the
structure of bump electrodes, the type of substrate, the type of addition
agent, and the type of conductive adhesive.
In each of the first to eighth embodiments, a Lewis-base compound, which
interacts more strongly with a free acid than a polar group at the surface
of a filler, is used as a rheology modifier. This rheology modifier not
only modifies rheology but also acts as a catalyst for reaction of
polyepoxide with acid anhydride. This improves the encapsulant's
resistance to various environmental conditions.
The first compare example is now discussed. This compare example uses an
encapsulant that is low in viscosity but high in thixotropy index and, as
a result of being high in thixotropy index, the encapsulant injection time
becomes long. This causes some interconnections to cut off upon
application of heat and thermal shock. Such failure may be caused by the
fact that when the encapsulant injection time is long the encapsulant
layer holds unwanted air bubbles, as a result of which non-uniform
application of thermal stress to the encapsulating layer occurs thereby
damaging conductive interconnections.
The second compare example is discussed. In this compare example, the
conductive adhesive used has high flexibility and the encapsulant used is,
for example, a high-viscosity resin of the phenol curing type epoxy resin
group. The encapsulant must be heated for the purpose of facilitating
injection. This causes some interconnections to have high connection
resistivity when the encapsulant is injected, and cutoff is likely to
occur where unstable interconnections exist in a thermal shock resistance
testing, since the encapsulant viscosity is high and the conductive
adhesive's junction is damaged by stress produced when the encapsulant is
injected.
The third and fourth compare examples are discussed. In these compare
examples, interconnections will cut off in a relatively short time. The
fourth compare example suffers great connection resistivity variation when
subjected to TESTS 4 and 5. In the third compare example, interconnections
fail to reduce thermal stresses and cutoff results. In the fourth compare
example, the encapsulant exerts strong thermal stresses and has high water
absorption, and cutoff results.
The fifth compare example is discussed. This compare example undergoes
great increase in connection resistivity when subjected to TESTS 1, 4, or
5. The reason may be that the anisotropic conductive adhesive's binder has
low humidity resistance, and low adhesion at high temperature. Use of an
anisotropic conductive adhesive formed of a binder having high humidity
resistance will cause interconnections to cut off when subjected to a
thermal shock test.
A semiconductor unit package in accordance with the present invention is
highly reliable against various environmental conditions. Conventionally,
encapsulant, which contain polyepoxide and acid anhydride (curing agent)
as a resin binder, have not been used in the flip-chip bonding method by a
conductive adhesive. If a resin binder made up of polyepoxide and acid
anhydride (curing agent) is used as an encapsulant for semiconductor unit
packaging, this increases the thixotropy index of the encapsulant,
therefore producing the problem that the encapsulant is injected to only a
part of a gap between the semiconductor device and the substrate.
The inventors of the present invention discovered that a high thixotropy
index results from interaction between a free acid contained in an acid
anhydride and a polar group at the surface of a filler. Based on this
knowledge, the present invention provides a means capable of impeding
interaction between free acid and polar group.
There is another reason why an encapsulant, which contains a polyepoxide
and an acid anhydride (curing agent) as a resin binder, has not been used.
That is, such a resin binder undergoes hydrolysis in a high humid
atmosphere, so that it has been considered that use of the resin binder
causes problems in humidity resistance of connections established by a
conductive adhesive, and in reliability.
It was confirmed by the present invention that even if a resin binder,
which uses an acid anhydride (particularly a trialkyltetrahydro phthalic
acid's anhydride) as a curing agent, is used as an encapsulant in a
flip-chip bonding step, a resulting encapsulating layer has sufficient
resistance to humidity to meet requirements of practical applications.
Additionally, such an encapsulant is low in viscosity and also low in
thixotropy index, so that even if the encapsulant is injected at room
temperature (low temperature), it well penetrates even into tiny gaps.
Such characteristics of the present encapsulant produce various
advantageous characteristics such as high resistance to thermal shock.
In the case of conventional semiconductor unit packages wherein in a
Flip-chip bonding step COMPOSITION f of TABLE 1 is used as a resin binder,
the encapsulant thixotropy index is so high that air bubbles are held in
the encapsulating layer. Conductive interconnections are damaged in TESTS
3 and 5. On the other hand, in the case of conventional semiconductor unit
packages wherein in a flip-chip bonding step COMPOSITION g of TABLE 1 is
used as a resin binder, the resin binder must be heated for injection. As
a result, conductive interconnections are damaged, and resistance to
thermal shock becomes low.
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